I've done enough Tacitus for now, so I thought I'd comment on your sentences.
(1) was generally nice, word order is perhaps an issue though. If you had swapped the order of multi consili and magnaeque audiciae, and then allowed magnae to succeed audaciae, you would have formed a neat chiasmus. You could have left imperator (in apposition as it is) later, and had:
Caesar audaciae magni multique consili ac rei militaris peritissimus erat imperator.
Nice vocab though.
for (2), for some reaso magnae multitudinis doesn't seem to work "of great (great) number". I think making magnus agree with the Germans and putting multitudo/magnitudo in the ablative may be more idiomatic. As regards the "nihil posse", that seems to me to be an over-simplication that even Tac. might shy away from. It is a good idea to introduce verbs instead of a weak adjective, but i think you need facere here.
Germani Romanos, cum magni multitudine essent, haud quicquam facere posse censuit.
(3) was v good. maybe bring sunt relecti to the end.
In (4) I saw what you were doing with the ppp data, but it's altering the sense somewhat, as her being given in marriage does not precede her beauty. Though "in matronium dare" seems to fit with English, collocare tended to be used more often and "in matrimonium" could even be removed ("filiam alicui nuptum" was generally used if dare was chosen). Seeing as there is talk of a king, the neighbouring "state" is probably a kingdom of sorts, so "civitas" (more 'body politic') may not be appropriate. Myabe regnum/imperium/terra, but finitimus is nice. Although you use odd accents, I imagine you mean the last 'a' of pulcherrima to be long, i.e. the ablative. I think it more idiomatic to keep facies in the ablative but have pulcherrima in the nominative. thus:
filia regis, a quo principi regni finitimi est collocata, femina facie pulcherrima erat.
In (5) you put circum after castra. Prepositions are so called for a reason
Other than in alterations in high-style verse, tenus and (often) versus are the only prepositions in prose to succeed their noun. You could, of course, have meant that the ditch constructed was "about nine feet", but that's not in the English. I know that 'fossam ducere' is a very nice way of saying "create a ditch/trench" and can only imagine that the same is good with perducere instead. nice. It may be better to back up your depth with altam, so that it is clear its depth, not length or width, is being decribed. A partitive genitive is not used after numbers here, but an accusative of respect, and numerals preceed what they qualify.
milites fossam novem pedes altam circum castra (per)ducent.
(6) was nice. nos hostesque would have bound them better, perhaps.