Yes but brand new 7sp triggers start at $30 PER PAIR. Chains $10, freewheels $11, cassettes $15... you can build a very nice new drivetrain for very little money. THe only downside is that new 7 speed is not available in the higher trim levels - XT etc. -- but for city use even the entry level stuffis very reliable. For used/NOS you are probably right - 8 and 9 is easier to find!

According to sheldon, 7speed spacing is 5mm and 8 speed is 4.8 - it will probably work but not be ideal for proper indexing - you have to work harder to keep it in tune.