There are a couple of parts here. The short answer is that you're still limited to illusion/enchantment spells when taking Rogue levels, but can copy any spell you can find (within level constraints) to your wizard spellbook.
Arcane Trickster spells
When you take a level in Rogue and get new spells, you can only choose spells from the Enchantment or Illusion schools (with exceptions at certain levels - see Thomas Markov's answer or PHB p.98 for the full text). These spells are not recorded in a spellbook. Per the multiclassing rules (PHB p.164), you determine the spells you know and prepare for each class individually. As a level 10 Arcane Trickster you'll know 7 spells of 1st or 2nd level.
When you take a level in Wizard, you can learn spells from any school of magic. Additionally, you can record spells from any school of magic in your spellbook, provided that they're of a level that you could cast as a wizard. As a level 6 wizard, these spells can be from 1st to 3rd level.
Multiclassing spell slots
As a wizard 6 / AT 10, you are effectively a 9th level spell caster. This gives you access to a pool of spell slots up to 5th level. Provided a large enough slot is available, these can be used to cast any spells you know, regardless of the school or which class you learned it from.