The rim on my tanks was fairly soft, I used a hacksaw blade and a pocketknife. If you are dealing with brittle or hardened plastics gat a $5.00 radio shack soldering iron (one of the small hobbyist irons not the big boys) Let it heat up, and carefully cut the plastic with the hot tip. I use this for egg crate and any other brittle plastic I deal with. It will stink a little, but the smell doesn't linger, and the ease and precision of cuting is worth it. with brittle plastics it is the only way to go IMO.
Just as a cautionary, molten plastic can and will cause severe burns due to the fact that it sticks and insulates while it burns your skin. Be careful You shouldn't have any dripping liquid plastic, but you will have some soft lava for a few seconds while cutting.
I have never had to cut more than the rim that the glass top sits on. thus the structure of the tank isn't compromised. and bear in mind that the tank rim is at least 50% asthetics, and doesn't (shouldn't) be a major support issue.
