You can assign individual keys/keystrokes with sliding

Well, I use a two-tap solution to get the '&': the umlaut key on the right hand accent panel and then it's the '6' on the number panel (or use the popup).

Regarding your idea with the slide I had no problem to assign a slide to produce a '&'. Tapping the 'i' menu I chose 'Custom Slides...' and then 'Define Letter'. In the 'Set Letter' box on top the 'a' is selected. I tapped 'p' (just as an example) and then chose a direction, in my case 'W'. Tapping the 'p' on the 'W'-line gives a dialog titled 'West from 'p''. Three tap later (Backspace - Umlaut - 6) I've got my '&'. 'OK' - 'OK' - 'Quit' and I'm done: sliding west from 'p' produces a '&'.

You may have to activate 'Caps & Custom' Slides under 'Sliding Options...'.

HTH,
- Juergen-