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Mark,
Using narrowband filters from different manufacturers wont make a difference. There's no channel cross-talk between SII, Ha or OIII filters. If there was, the filters wouldn't be narrowband, but broadband like conventional RGB filters. I know of people using a Custom Scientific 4.5nm Ha filter and use it with OIII and SII data acquired with 6nm filters. I'd use the SII data you've got. What I suspect you're trying to portray is that you haven't got enough SII data. This is a common problem with narrowband images as the SII filtered data typically requires much longer exposure than Ha and OIII. A weak SII channel is miserable to deal with, especially on SHO palettes, not too much of a problem with HOS. On occasion, I've blended Ha data into the SII to address this. Such an activity would upset the narrowband purist, but in the quest of producing a reasonable result, it works ok. Alex's advice is good. Many bin the SII and OIII data reducing the amount of time required to collect quality data. This however will depend on how you intend to process the image. With PS clipping masks, you process each channel to extract as much detail as possible, then combine. Its preferable to have unbinned data in this scenario. Binned works ok, but the features in each emission line will be weaker that the Ha lum. There is plenty of info in on processing narrowband images in other areas of these forums. A search will reveal them. Above all, don't be afraid of experimenting with the data you've got. In some cases, narrowband is easier to work with as there is no such thing as colour balance or at least not in the traditional sense of G2V calibration. Understanding the data's weakness will allow you to focus on what needs to be done to address it, be it acquiring more data for a specific channel or use alternate processing methods to extract the desired result. Keep at it.
Cheers
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