The blockchain takes up around ~230MB of disk space at the current height of 69600 blocks.
I think it's 145 Mb to download. The rest is indexes which your client will create from the 145 Mb it downloads.
Your wallet keys are basically converted to clam keys on import, and there's virtually no danger here unless your machine is compromised. The once imported data is not stored anywhere in the CLAM client, but that's not to say you shouldn't use multiple btc/ltc/doge wallets (you should). However, there's no way to tell if you own or owned the coin without full key pairs (unless I'm seriously missing something here).
I think you have this wrong.
The CLAM client stores your private keys, which are THE SAME as your Bitcoin private keys.
If someone gains access to your CLAM wallet (and the password, if encrypted), then they also have access to the BTC, LTC, or DOGE at the addresses you imported.
It's possible to convert from BTC addresses to CLAM addresses (and back again) without knowing the private keys. I posted a short Python script which does exactly that not long ago. See this post:
It's a small Python script that takes any number of BTC, LTC, DOGE, and CLAM addresses and gives links to the corresponding blockchain explorer pages for the equivalent BTC, LTC, DOGE, and CLAM addresses.