I'd add the XP and leave the 400s' on the tank. Considering the fish you have and your plans to add more, you will benefit from it.
Agreed!! Don't worry about extra-filtration. (there's no such thing as overfiltration IMO) Extra-filtration gives you more options for media, flexible maintainance & a safeguard in case of filter failure. You appear to want to run your tank at or close to max (by the time they reach adult). Additional filtration will be needed. XP3 is a good choice, as are Fluval 404/05, Cascade 1100/1500 or if you can stand it Eheim 2028.
Agreed!! Don't worry about extra-filtration. (there's no such thing as overfiltration IMO) Extra-filtration gives you more options for media, flexible maintainance & a safeguard in case of filter failure. You appear to want to run your tank at or close to max (by the time they reach adult). Additional filtration will be needed. XP3 is a good choice, as are Fluval 404/05, Cascade 1100/1500 or if you can stand it Eheim 2028.