  We study the family of isomorphisms and strictly singular operators in mixed
Tsirelson spaces and their modified versions setting. We show sequential
minimality of modified mixed Tsirelson spaces $T_M[(\mc{S}_n,\theta_n)]$
satisfying some regularity conditions and present results on existence of
strictly singular non-compact operators on subspaces of mixed Tsirelson spaces
defined by the families $(\mc{A}_n)_n$ and $(\mc{S}_n)_n$.
