Ah. Well if your cousin understands the rules and what you're trying to accomplish, it shouldn't be a problem. What I do with my friend is tell him my idea, and we give each other input on what we think the deck should have. We build it, and test it against one of the decks we've made, or a netdeck, and see what areas need improvement. Since we are both friends and we are just testing, we agree that proxies are okay, until we decide on a definitive list.
I would say that during league meetings, no, people might not accept playing with you if the deck is full of proxies. However, outside the league (or maybe before/after the event, during breaks or whatnot) they would be happy to. It's the spirit of the game, right? To help each other out? Although, I guess some people treat leagues more competitively, and want to be faced as such, and therefore feel proxies are unacceptable. This would just depend on the person. If you tell a judge what you're doing and why you're using proxies, they might let you keep playing or they might tell you to use all real cards during the league, but they won't shoot you down for wanting to test a deck idea. It's simply that during a league MIGHT not be the best place to use proxies in a experimental deck.
Netdecks, I learned of recently, haha. When someone mentions netdecking or something of the sort, it means they are playing or using a deck formed from a list they found on the Internet (hence the term, Inter"net"-deck). Many of these can be found in PokeGym's front page articles or sixprizes. Good players post up their decklists and strategies to share them and receive input. Popular ones are Vilegar and LuxChomp. I pretty much loosely label netdecking as anyone who doesn't use a deck they made but a deck they watched someone else make, so yeah.
For example, say your deck went successful. Someone saw the list of cards you had, and made the exact same deck, thinking of the exact same strategy. They would be considered netdeckers, netdecking your deck.
Personally, that idea with Meganium Prime crossed my mind a few times. I can't see Meganium as anything other than moving energy around, though. It's attack is okay-ish at most, so it could save you in a pinch as maybe a last-ditch effort or when you're stalling for something else. But as its own, as a main, I wouldn't give much hope to it. The idea that some have suggested with Leafeon is an idea that has come out a while ago. Meganium can serve a pretty vital role in that deck, allowing you to move your energies around. Shaymin for the HP is a nice add, but not exactly necessary, as it could be difficult to get out. If you do use it, Shaymin UL is a great choice, who allows you to move energy when you play it (I believe).
Meganium and Blissey is a decent combo, but definitely not in the sense that you should focus on either of them. I don't know your exact decklist, but look here on the 'Gym for deck staples. Many of them are so because they are so helpful in any deck. They may be able to speed things up.
Now, for the main attacker, I'm not going to decide that for you, that's up to whatever you want. However, my point still stands that Meganium/Blissey is a great support. Find trainers or supporters that can help you find constant energies and recycle and abuse Poke-POWERs, find a good strong Grass Pokemon that you can use to dish out damage or that you'd like, and follow a cycle in your tactics.
