Hi Helen,
It is not a dim question at all!
You certainly can't over filter, particularly where Botiine loaches are concerned.

The ratings that the manufacturers state are the maximum amount of water they are able to handle and this would be for a lightly stocked tank - the more fish, the lower the filter rating would become. As a rule, in-store, we tend to recommend half-rating what the maunfacturers claim (and incidentally we use one of Eheim's 600 litre filters on our River Tank, which has a volume of approx 200 litres).
If I were setting up a loach tank of that size, I'd opt for 2 of the 600 litre-rated models, which I think are the Professionel II 2028's. This would mean that you would have 6 media trays in total (so lots more room for ceramic biological media, mechanical sponge media, and if necessary, chemical media such as Rowaphos), and you can carry out maintenance on one filter at a time, to minimise disturbing all the filter bacteria in one go. You've also got the peace of mind that if one should fail for some reason (although not likely with Eheim models) you have back up with the other one. You could always start with one of these, and then add another at a later date, once you decide to start adding more fish. The loaches will certainly appreciate all the flow from these filters, and I'd also add a powerhead to increase circulation even further - you will see the obvious enjoyment on their part.
Our Clown tank is 1000 litres, and we have 3 Eheims on it, which have a combined total rating of 3000 litres (2 Professionel 3's rated at 1200 litres each, and one 600 litre filter). In addition to this we have two large Aquaclear 4000 powerheads on the tank. The water is visibly churning and the loaches love it.
With regards to the stocking levels of loaches, I totally agree with 5 being the absolute minimum number of each Botiine species, and more wherever possible.
I hope that this helps,
Emma