Read in this article 

https://www.ncbi.nlm.nih.gov/pubmed/21505980

when you cut the section, make sure you cut it longitudinal so that you get two equal parts 

You can cut the testes with a scapel, but like James John, cutting longitudinally allows you not only get two equal parts,  the cut allowsyou appreciate the true histological picture of the testes.

Cutting with scalpel is skilful operation.

Make sure that two equal parts reflect true histological picture.

Yes

If you don't mind i personally would prefer the use of a new, degreased, really sharp  razor blade (like e. g. former Gillette® Super Silver®, nowadays it seems to be supported only as: Gillette® Platinum-Plus® Double-Edge Blade ) guided between the tips of anatomical tweezers and used rather    in a sliding motion (only ONE TRACTION in the direction left to right [if you are right-handed])   than   "pressing and squeezing" the sample....if you do the sectioning in a big drop of physiological buffer solution  you then can transfer the halves quite easily to the fixative solutions of your choice. Concerning  your request   " suggest me the best fixative for the rat tissues." I frankly but honestly ask whether you are a philosophical student or a nevertheless a young researcher in Natural Science who is able to do some literature research or read specific books or eventually have the possibility to ask a PI (personal instructor) or attendant/team leader....This sentence you might argue is a bit harsh.... but usually we - in Natural /Life Sciences usually lay claim to work unaided/unassisted - at least to read a bit in the so called "ancient/ancillary" texctbooks of histology... do we? 

So if you mean only best fixative/fixative of choice for rat testis/testes it naturally -as usual - will depend on the analytical procedures you want to perform (i.e. plain histology = assessment of morphology after treatment  vs. control, [classical] histochemistry, IHC=Immunohistochemistry, CLSM (confocal laser scanning microscopy),  electron microscopy (SEM or TEM), etc., etc. ). If you meant rat 'tissues' you should specify in detail....and perhaps you'll get sufficient and specific reply to your problem... Best of luck  W.M. 

Disclaimer: not affiliated with the 'Gillette Company' and without a financial interest (only a satisfied consumer and for many years user of their product for specimen preparation of  human tissues for diagnostic purposes by TEM).