1st do I need a an overflow or can I use a bricklined spillway on ends of dam to control water level?
Take this as far as your budget will allow. As a rule of thumb, you pond size is adequte for your watershed, but every pond is different. I would put in a drain pipe with about 2 foot of freeboard for what little that would cost...just as insurance. Given a bigger budget...a bottom siphon system would be very nice to have.
Secondly what is the best initial grass for erosion control around the pond, brown top millet / winter rye mix?
I can't help with the specifics here, but get something going if you can. I got lucky and went through the first winter as the pond was filling with just bare dirt. Some washed down, but no ditches formed. I was fortunate. My dam has a great stand of crab grass and weeds now, even though I planted fescue and rye. The ground cover at my place rarely takes my advice, but I am grateful for it!
3rd the sides are really steep currently but the sides are held together with roots, vegetation. If we cut down the sides we will lose that built in erosion control. Open to any information. Thanks.
I assume you mean the inner slopes that make up the pond...if you don't want the extra size for the pond, IMO leave the natural cover. It will be less you have to worry about eroding during the fill. If there are trees roots involved. they will likely die once the water covers them up, depending on the tree type of course.
