Better soil than mine. I bought my plants maybe 9" tall, also
TSC. Set on 6 1/2" PT posts, with coated steel wire running across the top and earth anchors at either end. The 2' year old plants now run almost all 20'. Last year's plants (a thompson, and a flame seedless, from Lowes) are not muscadine varieties, and do not fair so well.
All the plants go dormant in winter, lose substantially all their leaves.
Pruning in early Spring definitely helps production. The roots of each grape should not be too close together - all that growth requires a lot of nutrients, you don't want them stunted due to excess competition. and yes, they get HEAVY. I'm using
a shed tie down kit (was about $30 cheaper when I bought it) for every two vines. Vines are spaced 10' apart, with 20' of suispended cable on which to train their growth. Had I been smarter, I would have alternated vines left to right, right to left, to further reduce resource competition. Next spring, I may set posts between the existing posts, and set more grapes, headed in the opposite direction, so I can keep 10' between root balls.
Muscadine like acidic soils, even more so than some of the other grape varieties - keep that im mind when you deep litter - and the chicken droppings will definitely help provide needed nitrogen. Build strong and be patient.