If you buy one of the "qualified" mixers and interfaces, you will get the complete Fat Channel Collection for free. Activations are multiple. For mixers it was said 3 activations, so there should be three or even more for Studio One. The Collection is registered to a user, not a hardware or software product.
The "Collection" is just equalizers and compressors, nothing else. They do, what compressors and equalizers do. There is no magic and no miracles are going to happen, most of the time you won't even hear what they do and "coloration" is anyway very subtile and very very subjective... Some of the "models" are already available in Studio One's Fat Channel, so you don't need to buy their "modeled" versions additionally. Some more are available for free, you just have to "buy" them in the shop and load them. It might make sense to look for plug-ins with visual control (compressors) and special sound processing (equalizers). I think two of the equalizers also have an Low Pass Filter.
If it would come down to two plug-ins to buy, go for the "Comp 160 Compressor" and "Alpine EQ-550" equalizer.