We have flown several times into Sanford with Allegiant. Nice small airport, very quick to get in and out of.
We have always been seated together, but we always make sure we are one of the first to check in as soon as the ticket stations open 2 hours before flight time. Make sure everyone is booked on the same itinerary. One of our sons decided to join us last minute a couple of years ago and ended up being seated halfway across the plane.
Friends of ours had some car trouble getting to the airport in January and arrived at the counter 30 minutes before flight time and ended up with seats all over the plane.
If your worried about being split up or don't want to be at the airport 2 hours before flight time, then I would suggest purchasing the seat assignment.
Pack light to help reduce the amount you pay in baggage fees. DW and I usually split a checked bag for our 7-10 days in Jan/Feb and just do a load of laundry half way through so we only have to pack 4 or 5 days worth of clothes.
The trip flex is OK if you are worried about having to cancel or change your vacation. For example if you are flying in hurricane season or if you have extra time available and you want to be able to extend your stay if you are having a really good time (or conversely ,book an earlier return flight if you are having a bad time or the weather is not co-operating).
If your vacation dates are not flexible and you feel that these are the dates that you need to fly and wont need to change, then you probably don't need the trip flex.
I always book my flight with a credit card that offers trip cancellation coverage, that way if the whole vacation goes sideways, I'm still covered