That's a natural reaction. I get completely overcome with panic/nerves when something happens to my fish too and I can't think straight.
I've had a look at Interpet's Guide to Fish Health which I have a copy of - great wee booklet. It mentions what products No.5 Liquisil General Tonic can be used simultaneously with but does NOT mention being able to be used in conjunction with other treatment products in the range. I've cross-checked against Interpet's No.9 Anti Internal Bacteria treatment and likewise, it looks as though it can't be used in conjunction with the general tonic. For that reason, I'd recommend a large water change prior to starting a new treatment.
As for going to MA, they tend to stock the Aquarium Munster range - MediMor would be the one to look out for. Your store may stock additional ranges too such as Interpet, Waterlife or eSHa.
Whichever you go for, probably best to do that water change immediately before dosing.
[Also, while you're there, it may be worth picking up a treatment for Whitespot, in the event of an unexpected outbreak some time down the line, and the need to dose immediately. One less thing to panic about / peace of mind if you have it in the house already.]