you need fusable- woven stablizer- it is fairly inexpensive- a woven lightweight fabric with fusable on one side- stablizes the knits so they do not stretch while sewing.
as for sashings---that is normal- since t-shirt decals are different sizes/shapes quite often sashings & blocks are used to make everything the same size for final construction- if you do a google images search of t-shirt quilts you will find a huge variety of them with all kinds of settings....there are probably a number of them to look at here on the board too- there are quite a few patterns for t-shirt quilts available also.