To my knowledge, there is no limit on newbies as far as friend requests go. You might have to be patient and just wait for things to develop. How long ago did you send out those friend requests? The reason I'm asking is because not everyone visits DPF every day and, perhaps, your recipients aren't yet aware of receiving your requests. It's also possible that your recipients might be waiting to find out a bit more about who you are and your personality/posting habits before they accept your friend request.
As far as the protocol for asking for trades goes, there really isn't one, per se. I would just go with a few guidelines and I think you'll be fine:
1. Always be polite throughout the whole process (initiating, negotiating, declining, accepting)
2. When you send out a trade request/email/private message, try to make the trade as fair as possible while understanding that people value pins differently (ask yourself if ownership of the pins in question were reversed, would you still consider the trade fair and want to make it)
3. Always use common sense and don't feel compelled to make every trade that comes your way; if an offer seems too good to be true, it probably is and never make a trade unless you are 100% comfortable with it
4. Don't forget to have fun - after all, isn't that why you collect pins?
After that, friendships will naturally fall into place
