Nothing really stops Lena from playing the field other than perhaps Ian. At least if you let Lena fall in love with him it might impact things I imagine. If you sleep with Ivy while doing Lena/Holly then Holly will have alternate dialogue comparing herself to Ivy so this might be a future pitfall too, but as long as you don't get Ivy and Holly involved you can sleep with her. Lena can also sleep with Mark. Doing the Holly relationships blocks off Emma for Lena, which is boring.
As for Ian Alison there are some pitfalls, but it depends on what version of Alison you want. I imagine you'd either want to actual romance or the sharing with Jeremy version and for that Alison you can't Emma and if you keep things with Lena, Holly or Cindy going with Ian then he'll break up with Alison automatically I think. I think you can sleep with Cherry once, the first time, where Alison will sleep with Jeremy and still get the romance/sharing route, but I haven't tried.
I think Ian can pursue Jess and Minerva independently from Alison. It remains to be seen how Louise and Ivy play into things.
Lena