Starling! I just saw your posts. Yes knits can be used but if your going to mount any armor to it directly you will probably have to reinforce it as most knits cannot support the weight without pulling or sagging.
In answer to your question about modification for later you need to think about armor mounting, temperature, and comfort in wearing. Also tailoring but I kinda figure you already realized that from some of you other posts.
If you are going to mount plates to the knit you'll probably need to reinfore that area. This can be done several ways. The fastest is to add iron on interfacing to the back of the area you are going to mount the plate. This will prevent the fabric from stretching (good and bad) allowing it to support the plates but now it won't stretch as well with you as you move. You can also line your pieces (Two layers of fabric instead of one). This however can get toasty depending on your fabrics. There are other methods but these are the most common ones that I have seen.
For my flight suit I have exercise (Yoga) pants and a rash guard (Both are stretchy moisture wicking materials that are on the sturdy side of kints). The trouble with this method is color matching.
I have made my own flight suit from a stretch suiting fabric and microsuede. It came out well but doesn't breathe as nice as my grey one. As far as patterns go I usually stick to horseback riding gear. Seriously, tight pants and shirts that are already designed with marks for altering built into the patterns and they are made for dense knits. Not sure what type of pattern you're useing but those are my recommendations.
Your thumb holes can definitely be added in the way you mentioned. Lengthening the sleeve and leaving and area of the seam open for the thumb. Just remember that the raw edges will need to be secured in some fashion. This can be as simple as a bit of bias tape or with great care, hemming the seam allowance in that hole.
Flight vests!!! The bane of my existence. Took me three tries before I got a tailored fit. I ended up altering a vest pattern several times. I used a high end pleather and a dense suiting knit for mine. I used the pleather on the central section of my vest to support the weight of my plates. My vest is also fully lined. Real leather was considered but I decided against it for three reasons. 1) temperature, leather is lovely to wear but it can get hot. 2) Sewing, multiple layers of leather can be hard to sew through. (my machine can't handle more then two thin layers and sewing by hand can murder the finger if you don't have the correct tools) 3) Price, leather is expensive unless you are thrift store shopping and get lucky with a leather jacket that you can successfully modify into a vest.
Quilted vests provide a bit of extra padding, the trade off is that quilted vests, by design, are meant to hold heat. This can be down played by a very thin lining layer and lightweight fabrics but somthing to consider.
Duck cloth is a common vest fabric. It is sturdy enough to support the armor on its own and breathes better than leather/pleather. Most people go this route. It also wears nicely, gets softer as you use it so that it move where you move (very stiff at first).
Sleeves or no...I went without true sleeves on my vest. The cap sleeves were too restricting when they were fitted. This was because of the fabric I chose. A good stretch will not do this but I was using my pleather. I ended up using a darted rectangle-ish shape to secure my shoulders. Again fabric and what type of shoulder your using will affect this.
Velcro is good when not visible. Most use sew on velcro, stays on better and can be removed and replaced when worn out. I personally hate the sound of velcro opening and closing and the constant 'crunchy' noise that it can make. So I only have two places that have velco that I have to open and close. (The shoulder on my vest and the front closure on my sash. The rest of the plates are secured using butterfly pins (I replaced the typical back with locking backs) and magnets.
One last piece of advise...When measuring yourself to determine your size on a pattern use the size that matches your largest measurement...it is easy to take fabric away much harder to add it in.
And remember to have fun!
