Carapaces Primed, Base Theme?

This weekend was a nice, three-day weekend for me. I took a day off of work on Monday for my birthday. Hooray, I'm old!
I managed to pick away at the attack bike I'm working on for a while over the weekend, and the bike itself is now done and sealed. I'll add some freehand icons to it later, when the Marines are done as well. I glued some small basing materials to the base (some skulls and rocks), and am at the blue highlight stage on the driver's backpack. The driver is primed and waiting til after the pack is done to start. I put a pin in the gunner's foot, and magnets in the guns. Everything is lined up to make sure I have the attack bike done for the FLGS's next tournament on the 16th...I hope.

Yesterday I managed to find an hour to airbrush prime the five Carnifex carapaces I have, in order to test some techniques and paint schemes. Airbrush priming white is far harder than black. Black is good to go in two coats. White took four or five.
I'm blaming that on two things: covering a darker color, and crappy pigmentation in the craft paint. I use Delta Ceramcoat for my black primer, but only one craft shop in the area carries the line anymore, so I settled for Apple Barrel at AC Moore instead of driving an extra 30 minutes. The Apple Barrel was not moving well through my airbrush. I couldn't seem to get a good thinning ratio. Eventually, I got all of the plates primed through sheer force of will and stubbornness. I just kept applying coat after coat of thin, splattery paint.
I need to either get some Delta, or experiment more with thinning the Apple Barrel stuff.

I'm planning to start with a Hive Tyrant, and will pick up the box as soon as I can get to the FLGS again. I might have to wait til the 16th.
The plan is to build him on foot, with all scything talons. I'd like to have him as dynamically posed as possible, but have to wait til I have the box in-hand, because I'm not familiar with how a Tyrant goes together. It might be a good idea for me to start fishing for spare Tyrant leg bits, just in case I screw up a reposing cut or something.

I'm also looking ahead to Genestealers. I know everyone thinks Genestealers are terrible in 6th, but I just really like the models and the aura of the unit. I'm trying to figure out two things: how to repose the legs to make them bounding or crouching, and how to build a Broodlord.
I'm trying to avoid having all of the models running forward, and instead want some leaping over rocks, skulking behind some cover, or coiling up for an attack. But the monopose legs of many of the smaller models makes that a challenge. Anyone ever reposed smaller Nid models?
The current Broodlord model is heinous. I don't want one of them in my collection, much less three or more. I have a copy of the newest Space Hulk edition, and the Broodlord model from that game. THAT is what a Broodlord should look like. I looked around, and the individual Broodlord model is upwards of $25 each. Ouch! Understandable considering its limited edition nature, but still, ouch.
So, I have to think of a way to make a new Broodlord, or three. Anyone ever made one that is larger and meaner than the regular Genestealers? I've seen the "conversions" online where folks just glue some spikes to the back of a regular Genestealer, and it doesn't do it justice.

Bases are also an important decision I need to make before I get too far into the project. My Marines are all on sandy bases painted and drybrushed various shades of brown. These days, I also try to scatter a small rock or some bitz and a tuft of static grass on the bases to break them up.
For Tyranids, a plain sanded base probably won't cut it, especially if I'm attempting to make them "high quality" models. I looked at cast resin bases from various manufacturers, but none really sparked my interest.
I'm thinking that small plants and tufts of field grass might be the best bet for the force. It'll add a small amount of color to the base, while not distracting from the models themselves. I've seen people use jungle grass and such for Nids in the past, but a lot of folks overdo it. The models end up obscured by the basing. As much as I'd love to do 3D bases, I also don't like seeing gaming models with integrated scenic bases, like standing ruins or columns. It's jarring on the table, and can cause minor in-game issues for line of sight and such. So, I'll likely create small custom plants from floral department items. You can do all sorts of neat things with floral wire and cut down cloth plants. I experimented with it briefly when working on terrain boards, but never got the practice off the ground. You can also get birch seed pods to use as leaf litter. A few leaves here and there should help add detail.
What should I use for ground cover, though? Sand and glue probably won't work, since I'm going for a loamy, forested approach. I've used Liquitex Texture Mediums in the past, namely Resin Sand. I used it as an additive to orange paint when making rusty pipes and tanks for a Mars pumping station board. It works nicely, and can easily be tinted with paint. The Resin Sand is a little too coarse for basing use. The particles are just too large. They do make a Natural Sand type, but I'm not sure how the texture would look in scale. I know GW makes those new texture paints for bases, but they're incredibly expensive for what they are, so those aren't an option.
Anyone have any suggestions?