Profit First is a cash management system (which helps you make lot more profit and take home a lot more money.
Xero is bookkeeping software used to track the in's and out's of your business money.
Profit First is not used for bookkeeping, it's used for cash management.
Xero is not used for cash management, it's used for historical bookkeeping and reporting.
All that means is that YES you can use Xero and Profit First (in fact that is absolutely the combo I recommend) - one does not hinder the other - in fact - they are both necessary (in my opinion) for a financially healthy company.