20lbs is quite a bit for a fluoro setup. You can use 2 x 4 s with the main cross beam being vertical rather than the weaker flatter orientation. You can even use pvc if you have to, but I would use electrical metal conduit inside of it to make it stronger. Be safe and hope all works out for you. And yes, you can use fluoros to veg. Many people have done it for veg and flower with good results.