I'm somewhat unsure about what (1) is good for.
In order to discriminate non-malicious UDP datagrams from some different protocol, you would in my opinion just use a different destination port number, that's by all means good enough. How many different UDP protocols do you have in use on your server anyway? Surely not 2,000 or 3,000 of them all at the same time -- it should be quite possible to use a port number (or a range of port numbers) that doesn't conflict.
On the other hand, prefixing a well-known 32 bit number isn't a very serious challenge for someone maliciously sending fake datagrams.
If anything, I'd negotiate a random per-connection ID and use that to quickly filter out the most stupid attackers. It's still no big challenge for someone who really means to send you fake packets, but at least it isn't totally trivial to circumvent, and it's more or less a no-op to verify (so it's a good initial check to comb out noise).
About (6), I'm also not sure. You primarily use UDP to have low latency. If you mean to send bulk data, you use TCP (easier, more reliable, and exactly as fast as UDP). Insofar, "large data" is somewhat contradictory. Large data cannot have low latency by definition.
I would, on the contrary, require that individual messages within datagrams have a size no larger than 255 bytes (serialization layer). This allows you to length-prefix them with a single byte, which is convenient and also prevents some denial of service attacks. If someone really needs to send several kilobytes or megabytes, they can still split up data at a higher level and send a few dozen/hundred messages, which will go into many datagrams.
Also, UDP/IP already supports fragmenting natively, in case one of your datagrams should really be too big.
(7) as proposed by hplus0603 is harder to get right than it sounds, but it is equally important. Nothing stinks more than "network that doesn't work" when nobody can tell for sure what exactly doesn't work or why. You really really really want good metrics when there are problems, both to make sure your software works as expected (during beta and stress testing) and for being able to tell a user who is complaining whether it's a problem at their end (and what they can do).