The 2 T-18's I have on order I ordered the belly heat, I also have RHP's I've already ordered and will install them when I get the T18's
From all I've read only the RHP's are required, I personally like to have a back up heat system in case of failure or issue with the RHP
The belly heat will give them a hot spot at least if needed.
I started my baby BP's out in a 10 gallon tank, it measured 20"x10" so a tub of that size should work
The first tub you posted was 14 3/16x 17 3/8 I would think that would work