I don't think a groundcloth or sealant is necessary. A silnylon bottom on a bivy is very durable and completely waterproof, it already has silicone impregnated into the fabric.
I would recommend a side zip bivy, it makes for easy bed entry and exit. I have slept in my bivy in temps from low 20's to high 50's and never had a problem getting too hot, however the nights in the high 50's I have pushed my quilt down to the bottom of the bivy and slept in only a T-shirt and shorts. A Momentum top bivy is remarkably breathable. It will add some warmth, which can be good if your trying to push a quilt thru colder temps. Again, dealing with the warm temps, while maintaining your bug protection and ground protection it not a big deal, you just adjust the amount of clothing and use of quilt accordingly.
I would recommend the MLD Superlight Bivy among any other. It's proformance, weight, and price are all a 10! It is extermely roomy, so when you hang it from the your tarp there is lots of head room, and no claustrophobic feeling.
Check it out here- http://www.mountainlaureldesigns.com/shop/product_info.php?cPath=22&products_id=30&osCsid=93d6c626a8ce79b4e2a4c301d35ef05e