That is something I'd use Securitates for as well. I personally just use Fusiliers as an universal proxy for similar Line Troops in other factions. If you spent the money on grabbing enough of Fusiliers/Zhanshies then repurposeing to represent other similar units is a good use of that money.
I use Bolts for just about everything since I've got like 9 of them and they're gorgeous. Mainly Order Sergeants (would recommend, they look great in a link with knights), Fusiliers in Vird and Swf, Bagh-mari, and the odd Merc unit if I'm feeling spicy. Now that Bolts are great I actually have a reason to use them as bolts. I intend to phase out the morat units I use whenever I play CA/Onyx with Shasvastii proxies once Defiance stuff comes in. Mainly Gwailos as Suryats, Jayths as Datrazi, and Cadmus as Rasyats, all shas units I either don't or cant use in Vanilla. May try calibans as rodoks in Onyx but that might be too much of a stretch. Don't get me wrong, Morats look great and all but I think they look better in their own sectorial. The hyper buff gorilla men just look out of place next to all the crooked legged bug robots and monsters.
Working on a project along these lines right now. Been buying minis from other factions to proxy over into JSA(I already have most everything for JSA already but wanted a "sub" set of proxied models to represent a group of people who defected into JSA from other factions). This project started from me nabbing the Zhanshis from the redveil/kaldstorm starters to use in my JSA, and I've been picking up some others as well. So far: Zhanshis from Redveil/Kaldstorm=Keisotsu Anaconda=Still is an Anaconda but being done in a JSA paint scheme Anaconda Pilot=being repurposed to a Tokusetsu Kohei O-yori Pilot(kickstarter version)=picked this one up to be the actual pilot for the Anaconda Penthislea=Aragoto Mymridon Hacker(from steel phalanx starter)=Aragoto dismounted Debating on getting a 3d printed drop ship for this squad so I can do a small diorama for them. *Edit* Decided to pick up a Saladin and use my extra Musashi model to proxy two Kempeis for this team to bring the point cost to a flat 200 to deploy them all together. It's not a balanced/competitive setup by any means, but was never meant to be I suppose either, was just more for the theme. Then for proxies I actually use for my legit factions. Dactyls Engineer-replaces Bakunin Clockmaker Hunzakut-proxies Druze Shock troop(I got the druze hacker and needed a second one to pair with her) Chandra Spec Op-Aragoto Dismounted Asura Multirifle(from aleph starter)-Kuroshi rider dismounted Senor Massacre=replaces Bran de Castro Zhanying Imperial Agent Hakcer-proxies authorized bounty hunter with sniper rifle Taureg(from redveil)=proxies ghulam with sniper rifle Think that's about everything I use proxies for. And yes...I buy entire starter sets just to use one mini(then give the rest of the minis to my brother, lol).