I will try to make a comprehensive answer on the subject.
First of all, Guapa(o), Hermosa(o), Linda(o), Bonita(o), Bella(o) are all synonyms to some extent, so one can be used in the place of the other most of the time. Although some are more appropriate than others depending on context, read further.
Second, let's define all of these terms according to the RAE, this alone may help you see the subtle differences between them.
bonito1.
(Del b. lat. boniton).
- m. Pez teleósteo comestible, parecido al atún, pero más pequeño.
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bonito2, ta.
(Del dim. de bueno).
adj. grande (‖ que supera a lo común). Tiene un bonito mayorazgo
adj. Lindo, agraciado de cierta proporción y belleza.
bello, lla.
(Del lat. bellus).
1. adj. Que tiene belleza.
- adj. Bueno, excelente.
□ V. arte bella bellas letras bello sexo
lindo, da.
(Del lat. legitĭmus, completo, perfecto).
adj. Hermoso, bello, grato a la vista.
adj. Perfecto, primoroso y exquisito.
m. coloq. Hombre afeminado, que presume de hermoso y cuida demasiado de su compostura y aseo.
lindo don Diego.
- m. coloq. lindo (‖ hombre que presume de hermoso).
de lo ~.
loc. adv. Lindamente, con gran primor.
loc. adv. Mucho o con exceso.
□ V. linda pesca linda pieza
hermoso, sa.
(Del lat. formōsus).
adj. Dotado de hermosura.
adj. Grandioso, excelente y perfecto en su línea.
adj. Despejado, apacible y sereno. ¡Hermoso día!
adj. coloq. Dicho de un niño: Robusto, saludable.
guapo, pa.
(Del lat. vappa, vino estropeado, hombre vil, vagabundo).
adj. coloq. Bien parecido.
adj. coloq. Animoso, bizarro y resuelto, que desprecia los peligros y los acomete. U. t. c. s.
adj. coloq. Ostentoso, galán y lucido en el modo de vestir y presentarse.
adj. coloq. U. en vocativo, vacío de significado, como expresión de cariño, a veces con retintín o con tono de irritación. Cállate un
poquito, guapo
m. Hombre pendenciero y perdonavidas.
m. En estilo picaresco, galán que festeja a una mujer.
m. pl. vulg. Prendas que se ponen en días de fiestas y ocasiones muy señaladas.
~ y apoyado.
- loc. adj. coloq. Ven. Dicho de una persona: Que tiene respaldo de los gobernantes.
Now that we defined those terms we can see that bonito is also a kind of fish, that lindo is also a man with female mannerisms or that guapo can be clothing or a guy that lies and is vain.
You ask if guapa can be used for both males and females, well guapa is inherently for females, but if you want it for males you have to say guapo. Guapa(o) is most of the time used for young people and specially for men (guapo, and depending on the region) Guapa can be used for females without it being weird or seldomly used but bonita for females is preferred over guapa. As guapo is preferred over bonito. Guapa(o) can be for someone that you wouldn't really treat formally even if you don't know this person. If you are young and she/he is young too, you would seldomly treat this person as usted, therefore you use guapo. Guapo is almost never used to describe a thing, except if you are teasing. (Please note that in many regions of Spain, guapo is to describe something that is cool, nice or awesome.) Nonetheless, in Latin America it is mostly used for people.
However if the person is someone that you would treat formally such as a old person, or a 'respected' person then you can probably say bello or bonito / lindo, now bello is more used in literary and in elaborate contexts or someone that wants to show vocabulary skills.
Hermoso is sometimes used to describe someone but more used to describe something in nature, a paysage or something that is very majestic. As the definition says hermoso is also used to describe a baby that is healthy in his looks, I can attest that this form is widely used as a native speaker.
For a more general word lindo or bonito is what you are looking for. Both words can be used in any context, to describe a person, young or old, formal or informal, to describe things or anything you can think of.
Nota: I am a native speaker and this is based mostly on experience with a grain of opinion. There are many regions that I don't know which words are more used than others, specially Central America, Philippines and Equatorial Guinea. Other than that I can say that this applies to the rest of regions to some degree.