[development] creating forward compatable modules?

[Tao Starbow]

Hi Drupal devs,

First, thank you all for an amazingly useful tool.

I am a programmer at the CITRIS center at UC Berkeley. We recently 
finished moving our own web site (http://www.citris-uc.org) over to 
Drupal and I am now working on the infrastructure to allow us to start 
providing Drupal-based sites for more of the campus community. So far I 
have been able to do everything I need with the available modules and a 
bit of hacking, but I am now starting to create custom modules.  I was 
wondering if it is possible to create modules that will be compatible 
with both 4.6 and 4.7?

Also, does anyone have advice on how to calculate how many Drupal sites 
a server can handle?  Installing new hardware can take months around 
here, so I need to stay well ahead of the curve.

thanks,
Tao Starbow
CITRIS