Downstem length need some advice

So a couple of months ago I bought a two foot beaker bottom and I recently dropped the downstem and broke the end off of it. The downstem that I broke touched the bottom of the beaker but just barely and it ripped. I thought that this was the correct length that the downstem was supposed to be because the friendly staff at Goodtimes Gallery in Burlington Vt set it up for me so i assumed that it was good. When I went to a different headshop closer to my home in Mass the other day to grab a new downstem the girl there told me that it was actually really bad that that the downstem touched the bottom of the bong at all saying that it could somehow break the bong so she advised me to choose the shorter length downstem telling me i could just compensate by adding more water if it was too short. The downstem she gave me was a 7" which I was skeptical about because it looked shorter than the original one I broke. When I got home I held the old downstem and the new downstem up next to one another and then measured them both with a ruler the new downstem was indeed 7" but the original was about 7 and 3/4" which perplexed me because the girl at the store was making it seem like downstems generally came in 5" 7" and 9" lengths. When I finally tried the new downstem it sat i would say about a half inch off of the bottom of the beaker and I did add a little more water than usuall but for some reason when I hit it it just kind of dragged and didn't rip as good as when I had the original downstem it especially dragged when the slide was out and I tried to clear it which was very frustrating. I tried using less water and it still dragged. The bong is a wee bit dirty though and has one set of percs so I was wondering if another possibiliity was that somehow some of the percs are gettin clogged up or gunky or something. Anyway I could use some advice on how to get my baby rippin again before I look to buy a sweet ashcatcher. BTW the width of the downstem is 18mm.

I look forward to hearing your thoughts thankyou