The LCC model is to operate with one common type, the A320 family being one type as far as crewing them are concerned, likewise the B737-6/7/8/9 family.
There have been exceptions to this rule, I've been in exile from Europe for 6 years now so perhaps out of date but recall Air Baltic opetating both Fokkers and Boeings, Air Berlin Fokkers and Airbus's and whatever else, Flybe, not strictly a LCC, operating BAE146's/Embraers of different sizes and DHC8's ... but then look at the recent difficulties Flybe have had etc.
Unless crews, particularly flight crews, are to be paid a across the board salaries for, let's say, C Series and Airbus's then the good ol' pilot seniority comes in to play, need to replace an Airbus pilot then need to upgrade and retrain a C Series pilot then need to employ and train a new C Series pilot to replace, doesn't fit in with the LCC business model, double the training costs etc. etc. etc whilst if the only standby aircraft/crew of a day might be A320 to replace a technical C Series ... A very good way all round for a LCC to lose money!