After the WoR, the Stone Lions were short on everything, warriors, equipment, everything. The SFE designs were targeted for the same reasons the Blood Spirits chose, durability & less resource cost. Assault, Heavy, and some Medium 'Mechs would go with SFEs for durability, trying to outlast their enemies, even if it means going slower. Fast Lights and Mediums would use an XL engine to get their speed up to the point they're hard to hit.
Shifting towards Protos is something a Clan can do when they have an excess of warriors, but not enough 'Mechs. The Lions don't have a ton of extra warriors so they wouldn't have to make a big shift to Protos, but they're not going to abandon Protos either. Being short on resources overall, the Stone Lions will keep using combined arms to take advantage of the easier training, production & repairs for vehicles over 'Mechs.
Initially, the Stone Lions are going to have a wide variety of units in use as they salvage everything they can.