Wow, such a long post! The truth is I don't develop FMP very much any more (X-Com Files is practically FMP 2.0), so won't do very broad changes, but I do add stuff occasionally and still care about it deeply, so input is always valuable.
Regarding alien races: it all comes down to randomness. Exactly like in vanilla, except FMP has a bit more races to choose from, so a particular race will appear less frequently on average.
In your example it looks like you didn't get the races that give psi (Sectoids, Ethereals, Cerebrals) - a rare, but possible and fully legitimate case. It means a no-psi campaign, sorry. But psi is not really necessary for winning, so better luck next time! (Same with Alien Electronics, which are mostly tied to the same races.)
Oh, and about the Blaster Launcher: it is blocked by psi because of how OP it is. Yes, it isn't mind-guided per se, but its interface is a bit more advanced than optronics and requires mastering psi engineering fore the circuits. Arbitrary, yes, but it was a conscious balancing decision.
Interceptors... I'll try rethink them, but there's not that much you can do - either you have the few techs that are related to them, or you don't. Breaking it would require complete redesign of everything, so it's unlikely at this point, but I'm open to suggestions.
MiBs are not very developed at this point - actually, it's Robin's initial mod with only three new units added - but I will work on it later. I'll certainly add a MiB Psi Ops which will also lead to psionics, in case you don't get psi aliens again.
But there were no changes as to how they work, so they didn't get any easier - randomness strikes again.
Weapon balance is a bit of a problem. OXCE+ gives you much more options to work with, but vanilla - which FMP is built for - is rather simplistic. But I am not really a fan of sniper rifles, they are good as a specialist weapon, but can't see how you could work it as a main weapon. Or maybe you could, but it would be totally anal to play. And shotguns are highly situational.
Too much Elerium? Sell it... What a weird complaint.
Gauss weapons use AP damage. It shoots bullets, but very very fast bullets. So fast they liquefy and evaporate quickly on their way (hence their limited range on crafts), but are very powerful despite low mass.
Railguns are a more advanced version; the are already in the mod, and many people consider it superior to plasma.
Advanced laser weapons have no clips and are overall better.
I just came here honestly to ask, has extendend inventory been tried with this mod? is it compatible? it says it may cause awkward alien behavior so i did not try it.
You mean Alien Armoury Expanded? Yes, it is included.
Cheers!