Whether or not spirit is your final career destination (LOLZ show me the money Bob) you really may be better off going through a full training course at another 121 outfit first. You'll get better first year pay at many regionals and be better prepared to either come to spirit or any other major carrier after that year. $35k-ish at Spirit vs. $50k at a regional, plus more comprehensive training before hitting the line... A bunch of my friends passed on Spirit and went to regionals instead because of this and it's really hard to find fault in their reasoning. They'll be flying for United/Delta/SWA/FedEx/UPS as quickly as anyone coming from Spirit and they'll make $20-$30k more along the way during their first couple of years. Some started a year ago and will be logging PIC time making CA pay before Spirit pilots see a new contract.
That said, if you study hard before even showing up for training, yea you can get through it with no prior 121 experience. The odds are against you and if your first ever V1 cut is when they show it to you in the sim before your checkride, then you're going to need to be well prepared to do that pilot shxt before you get here. Feeling lucky?