"Michael, does the Sawyer bottle have issues with a vacuum forming? Can the filter supplied with the bottle be used with the Squeeze pouch with an adapter? It looks a little smaller/lighter than the squeeze filter in the pictures."
John, no problem with a vacuum forming because there's a check valve thingy on the lid that lets air in.
I first tried the approach you asked about (adapting the smaller SP149 filter to act like a Squeeze). I attached a short length of tubing and an inline adapter to the "in" end of the filter, leaving the entire assembly of filter, tubing, and lid intact. This made for a rather unruly setup with flexible tubing on both ends of the filter. That's when I ordered the SP129 Squeeze (which amazon had for $35 and free shipping last week, but not as I write this).
Using the Squeeze filter simplifies the setup, in my opinion. And when I'm using the filter with the pouch I can just screw the two adapters together and put the lid back on the bottle to keep everything clean and minimize the potential for lost parts.