I can make a unit of 8 hastates / Princeps and 2 Triari?

NO you may create only units of 4 Triarii .So you need 2 or 3 units of Hastates counting at least 16 bases in total in order to have your unit of 4 triarii .

I am referring to mixed units. The units of hastati /princes are 4 6 8. In the notes it is said that the triari are 1/4 of the hastati/ princes and a unit of 4 astatine / principles + 1 triaro is shown At this point a unit of 8 could have 8 hastati / princes and 2 Triari in a mixed unit?

No only 5 bases allowed for the mixed unit. The "If combined, the hastati, principes and triarii form a TuG of 5." is an absolute so no alternative sizes allowed and is a special noted exception to the hastati & principes UG size of 4, 6 or 8 which only apply to non-mixed units.