I've done a bit of trenching, mostly for irrigation projects across sports pitches and have hired in a few types of trencher trying to find the perfect solution. Basically, what I've discovered is you need weight. Without it as soon as you hit a tough spot, the machine tries to either lift up and over the obstacle rather than go through it, or deviate on its path and you are left with a very wiggly trench. I've not tried one on a mini (what I assume you mean is a sub 3t machine) but I imagine it would act in a similar way to what the dedicated mini trencher did. And if your machine doesn't have a roller to power the aux, you will soon get a sore leg pressing on the pedal all day! They might well work ok on a 5 ton and up machine though?
Best success I have had is with a big tractor mounted one. Guy comes in for a day and does 4 times what I could do with the mini trencher or even just the bucket, and the arisings are collected in the trailer keeping the whole job very clean (important in my line of work). Expensive for a day yes - but the meterage achieved more than makes up for it and its the way I would recommend you go.