In my game, I started with lights and really had to earn my mediums. I've had a rough time of it since I ran Smithon, and one of my pilots in a Shadow Hawk was cored and killed (RIP Bullet Trap, we probably should have seen this coming). I'd already decided to scrap CT destroyed Mechs...and, in another complication, my heavy front line pilot (Behemoth in a Thunderbolt) took 4 injuries and went down for 90 days or so. Aaaand since I'd decided that Mechs were assigned to certain pilots, and couldn't be used otherwise, that left me to try and run missions with a stable consisting of a Jager, two Firestarters and a Wolverine.
It has been challenging as hell since Smithon! Every mission my pilots are fighting tooth and nail to overcome piles of heavies and mediums...so while I've picked up some more pilots and brought backup mechs out of storage, I'm constantly having to rotate out for repairs and injuries.
It's been plenty hard and fun for me with my little house rules!