My Mundo is the v.4 (latest version) and when I was looking all over for a front mounted seat, I could find all the photos of Mini Yepps, iBerts, and Bobikes but no one ever detailed how the installation went. Therefore, I just assumed the Mini Yepp would work easily. We had already tried the iBert and I didn't like the way it felt riding with it or the way Little Brother fit in it. It got in the way of my cables, scratched up the bike, didn't provide much back support, and the shoulder straps never seemed to stay up. The mount was difficult to take on/off and if the baby seat wasn't on, that mount looked like an injury waiting to happen. That said, some people love it. I don't.
What happened when I went to install my Mini Yepp mount, I quickly realized that my stem was too thick. Some internet searching later and I found out that it is not made for a threadless stem as it's too thick. I was incredibly frusterated because I had been so sure that it would work. I did some more searching and found an adaptor for mountain bikes/threadless stems but everywhere I looked, it was always out of stock in the USA. Jose took a look and figured out that the mount had spacers in it and when he removed the spacers, it clamped on the stem, no problem. That was fine until a day or so later, I noticed that the Yepp was sliding all around when we rode. Turns out, it was clamped well onto the spacers but the spacers were moving freely, round and round, taking Little Brother with them. Bummer. More research later (ie. reading customer reviews) and I saw that someone mentioned that they just took out the spacers and clamped the mount directly onto the fork. I took the bike into Practical Cycle and talked with Tim who said that would be fine. The mount would act as the spacers in that situation. We gave it a go and many miles later, the Mini Yepp is still holding strong. I kept the spacers for when I no longer need the mount, I'll still be able to put it all back together.
However, when I went to add the windshield, I found that there wasn't enough space between the mount and the stem so I then had to add a stem riser. Now, it's pretty perfect. It was a bit of a hassle to finally get here, but it does work. I love the Yepp seat, it's comfortable and lightweight. It's a better position for the little guy and for me to ride. The windshield is invaluable if you plan on riding regularly and gives you a foundation to put together a weather protector.
That said, the v.4 Mundo can come with either a threaded stem (quill) or a threadless stem. Jose is a bike snob and won't even consider a bike with threaded stem (he's got roadie mentality and I actually got a threadless stem by chance). I really recommend getting a threaded stem if you are going to go with a front mounted seat like a Mini Yepp. It will just make life easier and you'll be out riding with your wee one quicker without as many headaches as I had.