Q: How do you get apps to go on both devices(iPod and iPad) so that if I buy an app on my iPod it will straight away be there on my iPad?

You can turn on automatic downloads on both devices in Settings > Store : http://support.apple.com/kb/HT4539

Are the apps compatible with the other device e.g. do you have the relevant iOS version, are they iPad-only apps ?

What version of the operating system, iOS, have you got on the iPod and iPad (Settings > General > About > Version), and what minimum version do the apps require ? If, for example, the app needs iOS 5 but you only have version 4.3 on the iPod, then it won't download onto the iPod. Also some apps only work on the iPad (e.g. they need the iPad's larger screen) so they also won't download on an iPod. An app's requirements are listed on the left-hand side of their description page in the store.

I also assume that the device which the apps aren't downloading on is also connected to a network ?

In order to be able to download an app from the store the iPod and/or iPad need to be connected to the internet, either via wifi or (if the iPad is a 3G version) through 3G

Version 1.3 is the version of the iMovie app itself.

A bit lower down on the left-hand side of iMovie's description is a little heading that says Requirements - under which it includes : Requires iOS 5.1 or later. So in order to install iMovie on the iPod you will need to first update the iPod, in which case :

Updating to iOS 5 : http://support.apple.com/kb/HT4972

You need to be running iOS 5.1 on both devices anyway. The app will not work on your iPod or iPad unless you upgrade to iOS 5.1 on both devices.