Do check the timetable though, it's quicker to walk from Waterloo to Paddington and get a train from there than to go directly from Waterloo! It does make a change though and obviously Waterloo is much more conveniently situated.
On a point of pedantry, note that the Travelcard is for Zones 1-6, not just 1. Not that you'll be likely to use the extra validity
If you want both routes on the same ticket you will need ANY PERMITTED routing on the ticket, SLOUGH would be the Great Western route and STAINES would be the SWT route.
I think the only off-peak Travelcard is routed Any Permitted. There is no Slough routeing for walk-on tickets, and the route Staines off peak fares are only marginally cheaper anyway.